U.S. patent number 9,740,136 [Application Number 13/685,271] was granted by the patent office on 2017-08-22 for optical scanning apparatus and image forming apparatus.
This patent grant is currently assigned to CANON KABUSHIKI KAISHA. The grantee listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Katsuhide Koga.
United States Patent |
9,740,136 |
Koga |
August 22, 2017 |
Optical scanning apparatus and image forming apparatus
Abstract
An optical scanning apparatus according to one aspect of this
invention includes a light source that outputs a light beam having
a light power based on a supplied driving current, a detection unit
that detects the light power of the light beam, and a voltage
holding unit that holds a charged voltage used to control the
driving current. The optical scanning apparatus further includes a
control unit that controls a charging unit so that the voltage
holding unit is charged in a state where the driving current is not
supplied to the light source, and controls the charging unit based
on a detection result of the detection unit so that the voltage
held in the voltage holding unit is controlled from the voltage of
the voltage holding unit charged in the state where the driving
current is not supplied to the light source.
Inventors: |
Koga; Katsuhide (Moriya,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
N/A |
JP |
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Assignee: |
CANON KABUSHIKI KAISHA (Tokyo,
JP)
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Family
ID: |
47296953 |
Appl.
No.: |
13/685,271 |
Filed: |
November 26, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130147891 A1 |
Jun 13, 2013 |
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Foreign Application Priority Data
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Dec 8, 2011 [JP] |
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2011-269394 |
Nov 14, 2012 [JP] |
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2012-250587 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03G
15/043 (20130101); G03G 15/0266 (20130101) |
Current International
Class: |
G03G
15/043 (20060101) |
Field of
Search: |
;347/133,224,236,247,118,162 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101135876 |
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Mar 2008 |
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CN |
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07-171995 |
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Jul 1995 |
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JP |
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10093171 |
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Apr 1998 |
|
JP |
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H10-093171 |
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Apr 1998 |
|
JP |
|
H10-190155 |
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Jul 1998 |
|
JP |
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11-123845 |
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May 1999 |
|
JP |
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2001-024273 |
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Jan 2001 |
|
JP |
|
Other References
US. Appl. No. 13/572,246, filed Aug. 10, 2012. cited by applicant
.
U.S. Appl. No. 13/715,427, filed Dec. 14, 2012. cited by applicant
.
Chinese Office Action dated Sep. 2, 2014, in counterpart Chinese
Patent Application No. 201210524710.5, and English language
translation. cited by applicant.
|
Primary Examiner: Luu; Matthew
Assistant Examiner: King; Patrick
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. An image forming apparatus comprising: a photosensitive member;
a charger that charges the photosensitive member; an optical
scanning apparatus configured to scan the photosensitive member
with a light beam output from a light source when a driving current
modulated based on image information is supplied to the light
source; a developer configured to develop an electrostatic latent
image formed on the photosensitive member by scanning of the light
beam by the optical scanning apparatus to form an image on the
photosensitive member, and a control unit configured to control the
optical scanning apparatus, wherein the optical scanning apparatus
comprises: the light source configured to output the light beam
having a light power dependent on a value of the driving current; a
detection unit configured to detect the light power of the light
beam output from the light source; a capacitor configured to hold a
voltage; a charging unit configured to charge the capacitor, and a
comparison unit configured to compare a voltage corresponding to
the light power detected by the detection unit with a reference
voltage corresponding to the target light power, wherein the
control unit is configured to control the charging unit so that the
capacitor is charged by the charging unit and configured to control
the value of the driving current, wherein the control unit is
configured to control the charging unit so that the capacitor is
charged by the charging unit in a state where the driving current
is not supplied to the light source, to control the charging unit
based on a detection result of the detection unit so that the
voltage held in the capacitor is controlled from the voltage of the
capacitor charged in the state where the driving current is not
supplied to the light source, and to control the value of the
driving current based on the voltage held in the capacitor
controlled by the control unit, wherein the control unit is
configured to control the value of the driving current such that
the light power of the light beam is controlled to a target light
power, by changing the voltage of the capacitor based on a
comparison result of the comparison unit such that the voltage
corresponding to the light power detected by the detection unit
approaches the reference voltage, wherein the charging unit
comprises a switch configured to selectively apply one of the
reference voltage and the voltage corresponding to the detected
light power to the capacitor, and a resistive element connected
between the switch and the capacitor, wherein the charging unit is
configured to apply, in a turn-off state of the light source before
the light beam scans the photosensitive member, the reference
voltage to the capacitor during a period after charging of the
capacitor has started until the capacitor is charged to a voltage
corresponding to a predetermined amount of charges, and wherein the
period is determined depending on the reference voltage, a
predetermined voltage to which the capacitor is charged by the
charging unit, and a time constant determined by a capacitance of
the capacitor and a resistance value of the resistive element.
2. An image forming apparatus comprising: a photosensitive member;
a charger that charges the photosensitive member; an optical
scanning apparatus configured to scan the photosensitive member
with plural light beams; a developer configured to develop an
electrostatic latent image formed on the photosensitive member by
scanning of the plural light beams by the optical scanning
apparatus to form an image on the photosensitive member; and a
control unit configured to control the optical scanning apparatus,
wherein the optical scanning apparatus comprises: plural light
sources each configured to output a corresponding one of the plural
light beams, each of the plural light beams having a light power
dependent on a value of a corresponding one of plural driving
currents; a detection unit configured to detect the light power of
the plural light beams; plural capacitors each configured to hold a
voltage; plural charging units each configured to charge a
corresponding one of the plural capacitors; and plural comparison
units each configured to compare a voltage corresponding to the
light power detected by the detection unit with a reference voltage
corresponding to a target light power, wherein the control unit is
configured to control the plural charging unit so that the
capacitors are charged by the plural charging units and so that the
value of the driving current is controlled, wherein the control
unit is configured to control the plural charging units so that the
plural capacitors are charged by the plural charging units in a
state where the driving current is not supplied to the light
source, to control the plural charging units based on a detection
result of the detection unit so that the voltage held in each of
the plural capacitors is controlled from the voltage of the
capacitor charged in the state where the driving current is not
supplied to the light source, and so that the value of the driving
current is based on the voltage held in the capacitor, wherein the
control unit is configured to control the value of the driving
current such that the light power of each of the plural light beams
is controlled to the target light power, by changing the voltage of
the capacitor based on a comparison result of the comparison unit
such that the voltage corresponding to the light power detected by
the detection unit approaches the reference voltage, wherein the
control unit is configured to start to supply the driving current
to the light source after the capacitor is charged to a
predetermined voltage by the charging unit in a turn-off state of
the light source before the light beam scans the photosensitive
member, and when starting to supply the driving current to the
light source, the control unit is configured to decide the driving
current to be supplied to the light source based on the
predetermined voltage to which the capacitor is charged by the
charging unit in the turn-off state, wherein different driving
currents are supplied to the plural light sources, respectively,
wherein the plurality of charging units control voltages of the
plurality of different capacitors, respectively, and wherein the
optical scanning apparatus scans the photosensitive member by the
plural light beams output from the plural light sources.
3. An image forming apparatus for forming an image on a
photosensitive member with light beams, comprising: a plurality of
light sources each of which is configured to output a light beam
based on image data, the light beam having a light power dependent
on a value of a driving current; a detection unit configured to
detect light powers of respective light beams output from the
plurality of light sources; a plurality of capacitors respectively
corresponding to the plurality of light sources; a charging unit
configured to charge the plurality of capacitors; a control unit
configured to control the charging unit so that the plurality of
capacitors are charged by the charging unit, wherein the control
unit is configured to switch between a first mode for charging the
plurality of capacitors without using the detection unit and a
second mode for controlling the voltages of the plurality of
capacitors based on a detection result of the detection unit, and
the control unit is configured to cause the charging unit to
control the voltages of the plurality of capacitors by the second
mode after causing the charging unit to charge the plurality of
capacitors by the first mode; and a driving unit configured to
supply driving currents to the plurality of light sources based on
the plurality of capacitors respectively corresponding to the
plurality of light sources, wherein values of respective driving
currents supplied to the plurality of light sources are controlled
based on the voltages of the capacitors respectively corresponding
to the plurality of light sources, and the driving unit is
configured to supply to the plurality of light sources, based on
the image data, the driving currents respectively having the values
which are based on the voltages of the plurality of capacitors
controlled by the second mode.
4. An image forming apparatus for forming an image on a
photosensitive member with light beams, comprising: a plurality of
light sources each of which is configured to output a light beam
based on image data, the light beam having a light power dependent
on a value of a driving current; a detection unit configured to
detect light powers of respective light beams output from the
plurality of light sources; a plurality of capacitors respectively
corresponding to the plurality of light sources; a charging unit
configured to charge the plurality of capacitors; a driving unit
configured to convert a voltage of each of the plurality of
capacitors into a driving current having a value dependent on the
voltage and configured to supply the driving currents to the
plurality of light sources based on the plurality of capacitors
respectively corresponding to the plurality of light sources; and a
control unit configured to control the charging unit so that the
plurality of capacitors are charged by the charging unit and
configured to control the driving unit, wherein the control unit is
configured to, in a first period, cause the charging unit to charge
the plurality of capacitors without using the detection unit, and
cause the driving unit not to supply the driving currents to the
plurality of light sources, wherein the control unit is configured
to, in a second period subsequent to the first period, cause the
charging unit to control, based on a detection result of the
detection unit, the voltages of the plurality of capacitors charged
in the first period, and wherein the driving unit is configured to
supply to the plurality of light sources, based on the image data,
the driving currents respectively having values which are based on
the voltages of the plurality of capacitors controlled in the
second period.
5. An image forming apparatus for forming an image on a
photosensitive member with light beams, comprising: a plurality of
light sources each of which is configured to output a light beam
based on image data, the light beam having a light power dependent
on a value of a driving current; a detection unit configured to
detect light powers of respective light beams output from the
plurality of light sources; a plurality of capacitors respectively
corresponding to the plurality of light sources; a charging unit
configured to charge the plurality of capacitors, wherein the
charging unit charges the plurality of capacitors by a first mode
for charging the plurality of capacitors without using the
detection unit and charges the plurality of capacitors based on a
detection result of the detection unit by a second mode after the
charging unit charges the plurality of capacitors by the first
mode; and a driving unit configured to supply driving currents to
the plurality of light sources based on the image data, the driving
currents respectively having the values which are based on voltages
of the plurality of capacitors controlled by the second mode.
6. The image forming apparatus according to claim 3, further
comprising; a deflection unit configured to deflect the light beams
output from the plurality of light sources such that the light
beams output from the plurality of light sources scan the
photosensitive member, wherein the detection unit is configured to
receive the light beams output from the plurality of light sources
within a period other than a period for which the light beams
output from the plurality of light sources scan the photosensitive
member.
7. The image forming apparatus according to claim 4, further
comprising; a deflection unit configured to deflect the light beams
output from the plurality of light sources such that the light
beams output from the plurality of light sources scan the
photosensitive member, wherein the detection unit is configured to
receive the light beams output from the plurality of light sources
within a period other than a period for which the light beams
output from the plurality of light sources scan the photosensitive
member.
8. The image forming apparatus according to claim 7, further
comprising; a deflection unit configured to deflect the light beams
output from the plurality of light sources such that the light
beams output from the plurality of light sources scan the
photosensitive member, wherein the detection unit is configured to
receive the light beams output from the plurality of light sources
within a period other than a period for which the light beams
output from the plurality of light sources scan the photosensitive
member.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning apparatus and
an image forming apparatus that uses the optical scanning
apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus develops an
electrostatic latent image formed on a photosensitive member by a
toner and transfers and fixes the developed toner image to a
recording material, thereby forming an image on the recording
material. To form the electrostatic latent image on the
photosensitive member, the image forming apparatus uses an optical
scanning apparatus. The optical scanning apparatus includes a laser
light source that emits a laser beam, and a deflector such as a
rotating polygon mirror that deflects the laser beam emitted by the
laser light source so that the laser beam scans the surface of the
photosensitive member in a predetermined direction. To control the
light power of the laser beam scanning the surface of the
photosensitive member to a target light power, the image forming
apparatus executes APC (Automatic Power Control).
In the APC, the light power of the laser beam emitted by the laser
light source is detected using an optical sensor such as a
photodiode. A driving current to be supplied to the laser light
source is gradually adjusted such that the detected light power of
the laser beam reaches the target light power.
The APC includes initial APC executed as an initial operation for
making preparations for image formation and normal APC executed
during image formation. The normal APC is to control the light
power of the laser beam during, for example, the period of scanning
the surface of the photosensitive member. On the other hand, the
initial APC is to perform control to decide the value of the
driving current to be supplied to the laser light source in a
non-turn-on state as an initial operation when image data is input
to the image forming apparatus.
Japanese Patent Laid-Open No. 7-171995 describes the initial APC.
The light emission amount of a laser light source relative to a
supplied driving current changes depending on the temperature of
the light-emitting element or the time-rate change of the laser
light source. To prevent the laser light source from being damaged
by an excessive driving current supplied to it at the time of
initial APC, Japanese Patent Laid-Open No. 7-171995 discloses
initial APC that increases the driving current to be supplied to
the laser light source stepwise from 0, thereby controlling the
laser beam to the target light power.
However, since the initial APC described in Japanese Patent
Laid-Open No. 7-171995 executes the step of increasing the driving
current stepwise, a problem is posed that a control time that is
relatively long is necessary after the start of the initial APC
until the light power of the laser light source stabilizes near the
target light power, and image formation can be started.
In particular, in a multi-beam system using a plurality of laser
light sources, the initial APC is performed first for a specific
laser light source to be used to generate a synchronization signal
(to be referred to as a BD signal hereinafter) to define the image
write position. After the light power has approached the target
light power, the APC is started for the remaining laser light
sources. For the remaining laser light sources, the APC needs to be
performed at a timing so as not to cause the laser beam deflected
by a polygon mirror to expose the photosensitive member. To detect
such a timing, the light power of the laser beam to be used to
generate the BD signal needs to be adjusted to a light power that
allows BD signal generation. That is, after the initial APC has
been performed for the specific laser light source, the initial APC
is performed for the remaining laser light sources. Hence, the time
after the light power has been made to approach the target light
power by the initial APC until image formation can be started for
all laser light sources including the specific laser light source
and the remaining laser light sources further prolongs as compared
to the case in which a single laser light source is used. Hence,
there is deemed necessary a technique of shortening the time after
the start of initial APC until the light power of the laser light
source approaches the target light power.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the
above-described problem. The present invention provides a technique
of enabling the light power of a laser light source to approach a
target light power in a short time after turning on the laser light
source when executing APC in an optical scanning apparatus.
According to a first aspect of the present invention, there is
provided an optical scanning apparatus, for scanning a
photosensitive member with a light beam, comprising: a light source
configured to output the light beam having a light power dependent
on a value of a driving current; a detection unit configured to
detect the light power of the light beam output from the light
source; a voltage holding unit configured to hold a voltage; a
charging unit configured to charge the voltage holding unit; and a
control unit configured to control the charging unit so that the
voltage holding unit is charged by the charging unit and configured
to control the value of the driving current, wherein the control
unit controls the charging unit so that the voltage holding unit is
charged by the charging unit in a state where the driving current
is not supplied to the light source, controls the charging unit
based on a detection result of the detection unit so that the
voltage held in the voltage holding unit is controlled from the
voltage of the voltage holding unit charged in the state where the
driving current is not supplied to the light source, and controls
the value of the driving current based on the voltage held in the
voltage holding unit controlled by the control unit.
According to a second aspect of the present invention, there is
provided an image forming apparatus, comprising: a photosensitive
member; a charger that charges the photosensitive member; an
optical scanning apparatus configured to that scan the
photosensitive member with a light beam output from a light source
when a driving current modulated based on image information is
supplied to the light source; a developer configured to develop an
electrostatic latent image formed on the photosensitive member by
scanning of the light beam by the optical scanning apparatus to
form an image on the photosensitive member, and a control unit
configured to control the optical scanning apparatus, wherein the
optical scanning apparatus comprises: the light source configured
to output the light beam having a light power dependent on a value
of the driving current; a detection unit configured to detect the
light power of the light beam output from the light source; a
voltage holding unit configured to hold a voltage; and a charging
unit configured to charge the voltage holding unit, wherein the
control unit controls the charging unit so that the voltage holding
unit is charged by the charging unit and controls the value of the
driving current, and wherein the control unit controls the charging
unit so that the voltage holding unit is charged by the charging
unit in a state where the driving current is not supplied to the
light source, controls the charging unit based on a detection
result of the detection unit so that the voltage held in the
voltage holding unit is controlled from the voltage of the voltage
holding unit charged in the state where the driving current is not
supplied to the light source, and controls the value of the driving
current based on the voltage held in the voltage holding unit
controlled by the control unit.
According to the present invention, it is possible to provide a
technique of enabling the light power of a light source to approach
a target light power in a short time after turning on the light
source when executing APC in an optical scanning apparatus.
Further features of the present invention will become apparent from
the following description of embodiments (with reference to the
attached drawings).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view of an image forming apparatus
100 according to the embodiment of the present invention;
FIG. 2 is a view showing the arrangement of an exposure controller
10 according to the embodiment of the present invention and the
connection relationship between the exposure controller 10 and a
sequence controller 47;
FIG. 3A is a block diagram showing the arrangement of a laser
driving device 31 according to the embodiment of the present
invention;
FIG. 3B is a block diagram showing the arrangement of an APC
circuit 403 according to the embodiment of the present
invention;
FIG. 4 is a timing chart showing the light emission sequence of the
laser driving device 31 according to the embodiment of the present
invention;
FIG. 5 is a timing chart showing the relationship between an input
voltage and an output voltage Vsh of a hold capacitor 505 according
to the embodiment of the present invention;
FIG. 6 is a flowchart showing the procedure of an APC operation
executed for the laser driving device 31 according to the
embodiment of the present invention; and
FIG. 7 is a timing chart showing a comparative example of the light
emission sequence of the laser driving device 31.
DESCRIPTION OF THE EMBODIMENTS
Hereinafter, embodiments of the present invention will be described
in detail with reference to the accompanying drawings. It should be
noted that the following embodiments are not intended to limit the
scope of the appended claims, and that not all the combinations of
features described in the embodiments are necessarily essential to
the solving means of the present invention. Each of the embodiments
of the present invention described below can be implemented solely
or as a combination of a plurality of the embodiments or features
thereof where necessary or where the combination of elements or
features from individual embodiments in a single embodiment is
beneficial.
<Arrangement of Image Forming Apparatus 100>
The basic operation of an optical scanning apparatus and an image
forming apparatus according to an embodiment will be described
first with reference to FIG. 1. FIG. 1 is a schematic sectional
view of an image forming apparatus 100 according to this
embodiment.
In the image forming apparatus 100, documents stacked on a document
feeder 1 are sequentially conveyed onto the surface of a platen
glass 2 one by one. When the document is conveyed onto the surface
of the platen glass 2, a lamp unit 3 of a reading unit 4 is turned
on, and the reading unit 4 irradiates the document with light while
moving in the direction of an arrow 110. The light reflected by the
document passes through a lens 8 via mirrors 5, 6, and 7 and is
then input to an image sensor unit 9 and converted into an image
signal. The image signal output from the image sensor unit 9 is
temporarily stored in an image memory (not shown). After that, the
image signal is read out from the image memory and input to an
exposure controller 10.
The exposure controller 10 causes a laser light source to be
described later to emit a laser beam (light beam) to expose the
surface of a photosensitive member 11 (for example, photosensitive
drum) based on the input image signal (image information). The
photosensitive member 11 is scanned by the laser beam emitted by
the laser light source. When the photosensitive member 11 is
scanned by the laser beam, an electrostatic latent image is formed
on its surface. A potential sensor 30 detects the surface potential
of the photosensitive member 11 and simultaneously monitors whether
the surface potential has a desired value. A developer 13 develops
the electrostatic latent image formed on the surface of the
photosensitive member 11 by a toner. A transfer unit 16 transfers
the toner image developed by the developer 13 to the surface of a
recording material.
The recording material to which the toner image is to be
transferred by the transfer unit 16 is fed and conveyed from a
recording material stacking unit 14 or 15 in synchronization with a
timing at which the toner image reaches the transfer unit 16. The
recording material to which the toner image has been transferred by
the transfer unit 16 is conveyed to a fixing unit 17. The fixing
unit 17 fixes the toner image on the surface of the recording
material. After the fixing processing by the fixing unit 17, the
recording material is discharged from a discharge unit 18 to the
outside of the image forming apparatus 100.
After the transfer by the transfer unit 16 has been done, a cleaner
25 collects the toner remaining on the surface of the
photosensitive member 11, thereby cleaning the surface of the
photosensitive member 11. Next, an auxiliary charger 26 removes
charges from the surface of the photosensitive member 11 so that
the photosensitive member 11 can obtain a satisfactory charge
characteristic upon charging by a primary charger 28 at the next
time of image formation. In addition, after the residual charges on
the surface of the photosensitive member 11 are removed by a
pre-exposure lamp 27, the primary charger 28 charges the surface of
the photosensitive member 11. The image forming apparatus 100
executes image formation for a plurality of recording materials by
repeating the above-described processing.
<Arrangement Of Exposure Controller 10>
FIG. 2 is a view showing the schematic arrangement of the exposure
controller 10 according to this embodiment and connection between
the exposure controller 10 and a sequence controller 47. The
sequence controller 47 includes a CPU (not shown), and the CPU
controls the exposure controller 10 and the photosensitive member
11. As shown in FIG. 2, the exposure controller 10 includes a laser
driving device 31, a collimator lens 35, a stop 32, a polygon
mirror 33, an f-.theta. lens 34, and a BD (Beam Detect) sensor 36.
The laser driving device 31 includes a semiconductor laser (laser
diode (LD)) 43 including a plurality of light-emitting points for
emitting laser beams, and one photodiode (PD).
The operation of the exposure controller 10 based on the control of
the sequence controller 47 will be described next. The sequence
controller 47 included in the image forming apparatus 100 controls
the laser driving device 31 using a control signal S47 output to
the laser driving device 31. When image formation starts, the
sequence controller 47 controls each light-emitting point of the
semiconductor laser 43 to a turn-on state or a turn-off state based
on the control signal S47. Each laser beam emitted by the
semiconductor laser 43 is converted into a substantially collimated
light beam via the collimator lens 35 and the stop 32, and then
enters the polygon mirror 33 in a predetermined spot diameter.
The polygon mirror 33 has a plurality of mirror surfaces and
rotates in the direction of an arrow 201 at a uniform angular
velocity. Along with the rotation in the direction of the arrow
201, the polygon mirror 33 reflects each laser beam so that the
laser beams that have entered are deflected at continuous angles.
Each laser beam deflected by the polygon mirror 33 enters the
f-.theta. lens 34. The f-.theta. lens 34 applies a condenser effect
to the plurality of laser beams that have entered, and corrects
distortion to guarantee temporal linearity when the plurality of
laser beams scan the surface of the photosensitive member 11. The
plurality of laser beams scan the surface of the photosensitive
member 11 in the direction of an arrow 202 at a uniform
velocity.
The BD sensor 36 is a sensor used to detect a laser beam reflected
by the polygon mirror 33. The BD sensor 36 detects a laser beam
emitted by a specific light-emitting point out of the laser beams
reflected by the mirror surfaces of the polygon mirror 33. That is,
the sequence controller 47 controls the specific light-emitting
point so that the laser beam emitted by the specific light-emitting
point scans the BD sensor 36. Upon detecting the laser beam, the BD
sensor 36 outputs a synchronization signal (BD signal) S36
indicating the detection of the laser beam to the sequence
controller 47. The sequence controller 47 controls the turn-on
timing of each light-emitting point based on image data using the
BD signal S36 as a reference.
The sequence controller 47 monitors the period of output of the BD
signal S36 from the BD sensor 36, thereby monitoring the period of
laser beam detection by the BD sensor 36. In addition, the sequence
controller 47 controls to accelerate or decelerate a polygon mirror
driver (not shown) for driving the polygon mirror 33 such that the
period of one rotation of the polygon mirror 33 is always constant.
By this control, the sequence controller 47 sets the polygon mirror
33 in a stable rotation state.
<Arrangement of Laser Driving Device 31>
The arrangements of the laser driving device 31 and an APC circuit
403 (APC circuits 403-1 to 403-n) included in the laser driving
device 31 will be described next with reference to FIGS. 3A and 3B.
The arrangement of the laser driving device 31 will be described
first with reference to FIG. 3A.
The laser driving device 31 includes the semiconductor laser 43.
The semiconductor laser 43 includes a plurality of (n)
light-emitting points (LD1 to LDn) and one photodiode (PD). The
laser driving device 31 is also provided with the plurality of APC
circuits 403-1 to 403-n in correspondence with the plurality of
light-emitting points (LD1 to LDn).
The PD in the semiconductor laser 43 detects a laser beam from each
of the LD1 to LDn, and outputs a current Im corresponding to the
detected light power to a current/voltage converter 401. The
current/voltage converter 401 converts the received current Im into
a voltage and outputs it. An amplifier 402 is used to adjust the
gain of the voltage output from the current/voltage converter 401.
That is, the amplifier 402 adjusts the gain of the output from the
PD that has detected the laser beam from each of the LD1 to LDn.
The voltage that has undergone the gain adjustment by the amplifier
402 is supplied from the amplifier 402 to the APC circuit 403 as a
light power monitor voltage Vpd. Note that the PD, the
current/voltage converter 401, and the amplifier 402 in the
semiconductor laser 43 are provided to detect the light power of a
laser beam output from each light-emitting point.
The laser driving device 31 is controlled by the sequence
controller 47 based on various kinds of control signals included in
the control signal S47 output from the sequence controller 47, as
described above. The control signal S47 includes, for example, a
full turn-on signal FULL to be supplied to a logical element 412, a
control signal OFF_LD to be supplied to switches 408-1 to 408-n,
and control signals OFF_APC* (OFF_APC*-1 to OFF_APC*-n) and sample
hold signals S/H* (S/H*-1 to S/H*-n) to be supplied to the APC
circuits 403-1 to 403-n. The control signal S47 also includes a
light power control signal to be output to a current controller 506
to be described later.
The control signal S47 (the control signals OFF_APC* and the sample
hold signals S/H*) from the sequence controller 47 is input to the
APC circuits 403-1 to 403-n. In addition to the control signal S47,
a reference voltage Vref from the sequence controller 47 is input
to the APC circuits 403-1 to 403-n via digital/analog conversion
(D/A) circuits 417-1 to 417-n. The D/A circuits 417-1 to 417-n
convert a digital value representing the reference voltage Vref
input from the sequence controller 47 into an analog value and
input it to the APC circuits 403-1 to 403-n as the reference
voltage Vref, respectively. Under the control of the sequence
controller 47, each of the APC circuits 403-1 to 403-n performs
control to adjust the light power of a corresponding one of LDs
(LD1 to LDn) so as to cause the plurality of LDs (LD1 to LDn) to
emit light of a predetermined light power. Each of the APC circuits
403-1 to 403-n executes light power control of a corresponding LD
based on the reference voltage Vref in accordance with the control
signal S47 from the sequence controller 47.
A modulator 413 outputs, to the logical element 412, an image
modulation signal to be used to modulate driving currents to be
supplied to the LD1 to LDn using an image signal (image
information) input from an image signal generation unit (not shown)
or the like. For example, to perform PWM (pulse width modulation)
of a driving current, the modulator 413 outputs a pulse signal
having a width corresponding to image data to the logical element
412 as an image modulation signal. The logical element 412 outputs,
to switches 409-1 to 409-n, a signal representing the OR (logical
addition) of the image modulation signal output from the modulator
413 and the full turn-on signal FULL output from the sequence
controller 47.
As shown in FIG. 3A, the laser driving device 31 includes current
sources 404-1 to 404-n and 407-1 to 407-n for supplying (applying)
driving currents to the LD1 to LDn in the semiconductor laser 43.
The laser driving device 31 also includes the switches 408-1 to
408-n and 409-1 to 409-n that switch the current supply states from
the current sources to the LD1 to LDn. For example, the driving
current for the LD1 is supplied from the current sources 404-1 and
407-1, and the supply state is switched by the switches 408-1 and
409-1. The operations of the current sources 404-1 and 407-1 and
the switches 408-1 and 409-1 corresponding to the LD1 out of the
LD1 to LDn will mainly be described below. The description of LD1
also applies to the remaining lasers LD2 to LDn.
The switching current source 404-1 and the bias current source
407-1 for supplying a driving current to the LD1 are connected in
parallel between the power supply and the LD1.
The bias current source 407-1 supplies a bias current to the LD1.
The bias current is a current supplied to the LD1 to cause it to
emit a laser beam of a light power that does not change the
potential on the photosensitive member 11. When the switch 408-1 is
turned on, the bias current source 407-1 supplies the bias current
to the LD1. In a case in which the bias current is supplied to the
LD1, the time until the light power reaches the target light power
when supplying a switching current to be described below to the LD1
can be shortened as compared to a case in which no bias current is
supplied to the LD1. That is, supplying the bias current to the LD1
enables to improve the light emission responsibility of the LD1
when the switching current is supplied. In this embodiment, a laser
driving device for supplying a bias current having a predetermined
value to the LD1 will be exemplified for the sake of descriptive
simplicity.
The switching current source 404-1 supplies the switching current
to the LD1. The switching current is a current supplied to the LD1
to cause it to emit a laser beam of a light power that changes the
potential on the photosensitive member, and is supplied to the LD1
while being superimposed on the above-described bias current.
The APC circuit 403-1 controls the value of the current to be
supplied from the switching current source 404-1 to the LD1 by a
current control signal Isw-1 output to the switching current source
404-1. The switching current source 404-1 supplies a switching
current corresponding to the current control signal Isw-1 given by
the APC circuit 403-1 to the LD1 as a driving current. The switch
409-1 is connected between the LD1 and the switching current source
404-1. For this reason, driving current supply from the switching
current source 404-1 to the LD1 is set to the on/off state in
accordance with the on/off state of the switch 409-1.
The switch 408-1 is connected to the path from the switching
current source 404-1 and the bias current source 407-1 to the LD1.
The sequence controller 47 controls the switch 408-1 between the on
and off states using the signal OFF_LD output to the switch 408-1.
In this embodiment, if the signal OFF_LD output from the sequence
controller 47 is in the high state ("H"), the switch 408-1 is
turned off, and in the low state ("L"), the switch 408-1 is turned
on. If the switch 408-1 is in the on state, the switching current
source 404-1 and the bias current source 407-1 supply the currents
to the LD1. On the other hand, if the switch 408-1 is in the off
state, current supply from the switching current source 404-1 and
the bias current source 407-1 to the LD1 is cut off.
When the switch 408-1 is in the on state, and the switch 409-1 is
in the off state, the switching current is not supplied from the
switching current source 404-1 to the LD1, and the bias current is
supplied from the bias current source 407-1 to the LD1. Note that
the switch 409-1 is controlled to the on or off state based on a
signal supplied from the modulator 413 via the logical element
412.
When the switch 408-1 is in the on state, and the switch 409-1 is
in the on state, the bias current from the bias current source
407-1 and the switching current from the switching current source
404-1 are supplied to the LD1 as the driving current. In this case,
the LD1 outputs, to the surface of the photosensitive member 11, a
laser beam of a light power necessary for forming an electrostatic
latent image on the surface.
<Arrangement of APC Circuit 403 (403-1 to 403-n)>
The arrangement of the APC circuits 403-1 to 403-n included in the
laser driving device 31 will be described next with reference to
FIG. 3B. Each of the APC circuits 403-1 to 403-n performs APC for a
corresponding one of the LDs (LD1 to LDn). For the sake of
descriptive simplicity, APC by the APC circuit 403-1 for the LD1
will only be explained below. For the remaining lasers (LD2 to LDn)
as well, the APC can be implemented by performing the same control
as that of the LD1. Since all the APC circuits 403-1 to 403-n have
the same arrangement, the APC circuits 403-1 to 403-n will be
referred to as the APC circuit 403 hereinafter.
As described above, the reference voltage Vref corresponding to the
target light power of the LD1 and the light power monitor voltage
Vpd output from the amplifier 402 are input to the APC circuit 403.
In addition, out of the control signal S47 output from the sequence
controller 47, the control signal OFF_APC* and the sample hold
signal S/H* are output to the APC circuit 403. In the APC circuit
403, the reference voltage Vref is supplied to an analog switch 501
and the current controller 506. The control signal OFF_APC* is
supplied to the analog switch 501 and a logical element 502. The
sample hold signal S/H* is supplied to the logical element 502.
The light power monitor voltage Vpd and the reference voltage Vref
are input to the input side of the analog switch 501. One of the
light power monitor voltage Vpd and the reference voltage Vref is
output from the output side of the analog switch 501 as an output
voltage Vpd2 based on the control signal OFF_APC* from the sequence
controller 47. More specifically, if the control signal OFF_APC* is
"H", the analog switch 501 outputs the light power monitor voltage
Vpd as the output voltage Vpd2. If the control signal OFF_APC* is
"L", the analog switch 501 outputs the reference voltage Vref as
the output voltage Vpd2.
The logical element 502 is an element that outputs a signal
generated by obtaining a signal representing the AND (logical
product) of the received control signal OFF_APC* and sample hold
signal S/H* and inverting the logic of the obtained signal
(H.fwdarw.L or L.fwdarw.H), and corresponds to a NAND circuit. The
signal output from the logical element 502 is supplied to an analog
switch 504 as a control signal SEL.
The analog switch 504 functions as a sample hold circuit. The
output voltage Vpd2 of the analog switch 501 is applied to the
input side of the analog switch 504 via a resistive element 503.
The analog switch 504 switches between a sample state and a hold
state by switching based on the control signal SEL supplied from
the logical element 502 whether to output, from the output side,
the voltage input from the input side.
More specifically, if the control signal SEL is "H", the
output-side terminal and the input-side terminal connected to the
output-side terminal of the analog switch 501 are connected in the
analog switch 504. The analog switch 504 thus outputs, from the
output side, the voltage applied from the analog switch 501 to the
input side via the resistive element 503. On the other hand, if the
control signal SEL is "L", the analog switch 504 opens the input
side (the input-side terminal on the unconnected side is connected
to the output-side terminal).
When the control signal SEL is "H", the output voltage Vpd2 of the
analog switch 501 is applied to a hold capacitor 505 via the
resistive element 503. The hold capacitor 505 is charged by a
predetermined time constant .tau. when the voltage Vpd2 is applied
to it. The hold capacitor 505 changes the voltage in accordance
with the amount of charges accumulated by charging. In the turn-on
state in which the LD1 is on, the hold capacitor 505 outputs a
voltage corresponding to the light power monitor voltage Vpd. When
the control signal SEL switches to "L", the input side of the
analog switch 504 is opened, and as a result, the voltage of the
charged hold capacitor 505 is held.
As described above, the analog switch 504 and the hold capacitor
505 are set in the sample state when the control signal SEL is "H",
or in the hold state when "L". A voltage Vsh of the charged hold
capacitor 505 is input to the current controller 506. Note that the
time constant .tau. when charging the hold capacitor 505 is defined
as .tau.=RC depending on a resistance value R of the resistive
element 503 and a capacitance C of the hold capacitor 505. When
executing the APC, the hold capacitor 505 in the sample state is
charged to a predetermined voltage Vt in the turn-off state in
which the LD1 is off, or charged to the light power monitor voltage
Vpd in the turn-on state in which the LD1 is on, as will be
described later.
When the hold capacitor 505 is in the sample state, one of the
reference voltage Vref and the light power monitor voltage Vpd
corresponding to the light power detected by the PD in the
semiconductor laser 43 is applied to the hold capacitor 505 in
accordance with switching by the analog switch 501. That is, in
this embodiment, the analog switch 501 functions as a switch for
selectively applying one of the reference voltage Vref and the
light power monitor voltage Vpd to the hold capacitor 505. The
resistive element 503 functions as a resistive element connected
between the switch and the hold capacitor 505. Additionally, in
this embodiment, the analog switch 501, the resistive element 503,
and the analog switch 504 function as a charging unit.
The current controller 506 decides the value of the switching
current Isw based on the received reference voltage Vref and the
voltage Vsh of the hold capacitor 505. The current controller 506
outputs the current control signal Isw corresponding to the decided
value of the switching current Isw to the switching current source
404 (404-1 to 404-n). More specifically, when the LD1 changes from
the turn-off state to the turn-on state, and optical scanning of
the photosensitive member 11 by the laser beam output from the LD1
starts, the APC circuit 403 controls the voltage of the hold
capacitor 505 in the following way. That is, the APC circuit 403
controls the driving current to be supplied from the switching
current source 404-1 to the LD1 using the predetermined voltage Vt
generated in the turn-off state as the initial value, thereby
controlling the voltage of the hold capacitor 505. The current
controller 506 designates the driving current to be supplied from
the switching current source 404-1 to the LD1 by outputting the
decided switching current value Isw (Isw-1) to the switching
current source 404-1.
As described above, the hold capacitor 505 functions as a charge
accumulation unit which causes the laser light source (LD) to
output a laser beam of a light power corresponding to the
accumulated charge amount. That is, the hold capacitor 505
functions as a voltage holding unit which outputs a voltage
corresponding to the accumulated charge amount. The current
controller 506 and the switching current source 404-1 function as a
current supply unit which supplies a driving current corresponding
to the voltage of the charge accumulation unit (hold capacitor 505)
to the laser light source (LD) when optical scanning of the
photosensitive member 11 starts. The current controller 506 also
functions as a control unit which controls the voltage of the
charge accumulation unit (hold capacitor 505).
<Comparative Example of APC in Laser Driving Device 31>
A comparative example of APC in the laser driving device 31
according to this embodiment will be described next with reference
to FIG. 7. For the sake of descriptive simplicity, APC by the APC
circuit 403 (APC circuit 403-1) for the LD1 will only be explained
below. For the remaining lasers (LD2 to LDn) as well, the APC can
be implemented by performing the same control as that of the
LD1.
When executing APC for an LD included in the laser driving device
31, if the light power of the LD is controlled after turning on the
LD in the turn-off state, a considerable time may be necessary
until the light power sufficiently approaches the target light
power. FIG. 7 shows an example of the light emission sequence of
the laser driving device 31 as a comparative example to the
embodiment to be described below. In FIG. 7, an operation mode
including APC to be performed before the image forming apparatus
100 starts image formation will be referred to as an "initial APC
mode", and an operation mode including APC to be performed after
image formation will be referred to as a "normal APC mode". FIG. 7
shows the light emission sequence for two LDs (LD1 and LD2) out of
the LDs included in the laser driving device 31. The LD1 is an LD
used to detect a BD signal and is assumed to be an LD for which the
APC is executed first out of the plurality of LDs.
Referring to FIG. 7, first, to start the APC of the initial APC
mode, the sequence controller 47 switches the full turn-on signal
FULL of the LD1 from "L" to "H" to turn on the LD1. In addition,
the sequence controller 47 switches the sample hold signal S/H*
(S/H*-1) of the LD1 from "L" to "H" to shift to a state to sample
the light power of the LD1 detected by the PD. In this state, the
detected light power of the LD1 gradually increases. This is
because the sequence controller 47 controls the driving current to
be supplied to the LD1 such that the detected light power of the
LD1 approaches the target light power.
More specifically, the light power monitor voltage Vpd
corresponding to the light power of the LD1 detected by the PD in
the semiconductor laser 43 is input to the APC circuit 403. If the
APC circuit 403 is in the sample state, the hold capacitor 505 is
charged to the light power monitor voltage Vpd. The current
controller 506 compares the light power monitor voltage Vpd
generated in the hold capacitor 505 with the reference voltage Vref
corresponding to the target light power. In addition, the current
controller 506 decides the value of the switching current Isw based
on the comparison result such that the light power monitor voltage
Vpd approaches the reference voltage Vref. The value of the
switching current Isw is output from the APC circuit 403 to the
switching current source 404-1 as a current control signal (Isw-1).
The switching current source 404-1 supplies the switching current
Isw having a value corresponding to the current control signal
(Isw-1) to the LD1. During the sample state, the APC circuit 403
continuously controls the switching current value Isw based on the
light power monitor voltage Vpd and the reference voltage Vref. The
sequence controller 47 thus controls the light power of the LD1 to
the target light power using the APC circuit 403.
When the light power of the LD1 has sufficiently approached the
target light power, and it has become possible to stably detect the
BD signal, the sequence controller 47 ends the initial APC mode and
shifts to the normal APC mode. When APC of the normal APC mode
starts, the sequence controller 47 sets the LD1 in a full turn-on
state for a predetermined period Ts and samples the light power
every time a BD signal is detected (in every scanning). The
sequence controller 47 thus executes the APC by controlling the
driving current to the LD1 such that the light power of the LD1
approaches the target light power, as in the above-described
initial APC mode. The light power of the LD1 has been made to
sufficiently approach the target light power by the APC of the
initial APC mode. Hence, in the APC of the normal APC mode executed
after the initial APC mode, the light power of the LD1 can be made
to reach the target light power by several times of APC executed
every time a BD signal is detected.
In the APC of the initial APC mode described above, however, the
driving current of, for example, the LD1 is gradually increased
from 0, thereby gradually making the light power of the LD1
approach the target light power. For this reason, a relatively long
time T1 is necessary until the light power of the LD1 sufficiently
approaches the target light power and it becomes possible to stably
detect the BD signal, as shown in FIG. 7.
In addition, a longer time is necessary for the LD2 after the
driving current is supplied to turn on the LD2 until its light
power sufficiently approaches the target light power. As shown in
FIG. 7, after the shift from the initial APC mode to the normal APC
mode, the sequence controller 47 switches the full turn-on signal
FULL of the LD2 from "L" to "H" to turn on the LD2. In addition,
the sequence controller 47 switches the sample hold signal S/H*
from "L" to "H" to sample the light power of the LD2, and performs
control to make the light power of the LD2 approach the target
light power, thereby performing the APC of the LD2. After that,
light power control of the LD2 is repetitively performed next to
the light power control of the LD1 at a period Tb of BD signal
detection.
In this manner, after the APC of the initial APC mode for the LD1
has ended, the APC for the LD2 is performed in the normal APC mode
by performing control to make the light power of the LD2 gradually
approach the target light power from the turn-off state. For this
reason, the time until the light power of the LD2 reaches the
target light power is longer than that of the LD1. Hence, in the
image forming apparatus of the multi-beam system that exposes the
photosensitive member by laser beams emitted by a plurality of LDs,
the time until the light powers of all of the plurality of LDs are
controlled to the target light power by the APC (initial APC mode
and normal APC mode) becomes longer as a whole. For example, a time
T2 necessary after the light power control of the LD2 has started
until the light power reaches the target light power is
approximately Tb.times.T1/Ts. For example, assume that T1=10 [ms],
Ts=10 [.mu.s], and Tb=500 [.mu.s]. In this case, T2=500 [ms]. In
the image forming apparatus of the multi-beam system, when the
number of LDs increases, the time until the light powers of all LDs
reach the target light power prolongs in proportional to the number
of LDs.
The image forming apparatus according to this embodiment, when
executing APC of the initial APC mode for the laser driving device
31, enables light power control to starts from a light power close
to the target light power in order to make the light power of the
LD approach the target light power in a short time after turning on
the LD. More specifically, the hold capacitor that holds the
voltage used to cause the LD to output a laser beam is charged in
advance to a predetermined voltage close to the reference voltage
for the target light power during the turn-off state (before
turning on) of the LD before the start of optical scanning of the
photosensitive member 11. That is, charges in a predetermined
amount corresponding to the predetermined voltage close to the
reference voltage for the target light power are accumulated in the
hold capacitor during the turn-off state of the LD before the start
of optical scanning. The voltage of the hold capacitor is used to
decide the driving current to be supplied to the LD based on the
result of comparison with the reference voltage. In this
embodiment, since the hold capacitor has been charged in advance to
the voltage close to the reference voltage when turning on the LD
and starting the light power control of the LD, the LD can be
turned on in a light power close to the target light power at the
start of APC of the initial APC mode. This allows the light power
of the LD to reach the target light power in a short time by the
APC of the initial APC mode and the normal APC mode.
This embodiment assumes an image forming apparatus of the
multi-beam system. In the image forming apparatus of the multi-beam
system, for each of the LDs, charges in a predetermined amount are
accumulated in a corresponding hold capacitor during the turn-off
state before the start of optical scanning, thereby charging the
hold capacitor to a predetermined voltage. This allows all LDs to
make the light power reach the target light power in a short time
by the APC after turn on. Processing executed for the laser driving
device 31 in this embodiment will be described below in more
detail.
<APC in Laser Driving Device 31>
APC in the laser driving device 31 according to this embodiment
will be described next with reference to FIG. 4. For the sake of
descriptive simplicity, APC by the APC circuit 403 (APC circuit
403-1) for the LD1 will only be explained below. For the remaining
lasers (LD2 to LDn) as well, the APC can be implemented by
performing the same control as that of the LD1.
In the image forming apparatus 100, the APC executed for light
power control of each of the LD1 to LDn is divided into APC of the
initial APC mode and APC of the normal APC mode, as described
above. The initial APC mode is an operation mode including APC to
be performed as a preparation operation before the image forming
apparatus 100 starts image formation. In the APC of the initial APC
mode, control is performed from a complete turn-off state of each
LD such that the light power of the laser beam emitted by each LD
approaches the target light power. The normal APC mode is an
operation mode including APC to be performed after the start of
image formation. In the APC of the normal APC mode, the light power
of the laser beam emitted by each LD to expose the photosensitive
member 11 is controlled to the target light power.
The initial APC mode of this embodiment includes an initial
charging operation of charging the hold capacitor 505 to the
predetermined voltage Vt in the turn-off state in which each LD is
off before the start of driving current supply to each LD. The
initial charging operation need only be executed, for example, at
the time of activation of the image forming apparatus 100 or at the
time of a preparation operation before the start of formation of an
image to be transferred to a recording material. Assume here that
the initial charging operation is executed at the time of a
preparation operation of the image forming apparatus 100.
In the initial APC mode of this embodiment, the APC to control the
light power of each LD to a light power near a predetermined target
light power is executed in the turn-on state in which each LD is
on, after the initial charging operation has ended and driving
current supply to each LD has started. In this APC, when supply of
the driving current (switching current) to each LD starts, the hold
capacitor 505 is charged from the voltage Vt to the light power
monitor voltage Vpd corresponding to the light power detected by
the PD. In addition, the switching current is controlled based on
the result of comparison between the reference voltage Vref and the
light power monitor voltage Vpd generated in the hold capacitor
505. In this APC, the light power monitor voltage Vpd is controlled
to approach the reference voltage Vref from not voltage=0 but the
voltage Vt close to the reference voltage Vref corresponding to the
target light power, as will be described later. That is, control of
the driving current (light power) based on the light power monitor
voltage Vpd (corresponding to the light power of each LD) is
started from the voltage Vt close to the reference voltage Vref,
thereby controlling the light power of each LD to the target light
power in a shorter time. After that, when the image forming
apparatus 100 has started image formation, the initial APC mode
changes to the normal APC mode, and APC of the normal APC mode is
executed at a predetermined timing. The initial charging operation
in the initial APC mode and the APC of the initial APC mode and the
normal APC mode will be described below in detail in accordance
with the light emission sequence shown in FIG. 4.
(Initial Charging Operation in Initial APC Mode)
In the initial state before the start of image formation in the
image forming apparatus 100 (before a time 421 in FIG. 4), the
sequence controller 47 outputs the signal OFF_LD of "H". In this
state, the switch 408-1 is off, and the bias current and the
switching current to the LD1 are not supplied. Hence, since the LD1
is in the turn-off state, the light power monitor voltage Vpd input
to the APC circuit 403 is 0. Additionally, in the initial state,
the sequence controller 47 outputs the sample hold signal S/H* of
"H" and the control signal OFF_APC* of "H" to the APC circuit
403.
At the time 421, the sequence controller 47 changes the control
signal OFF_APC* from "H" to "L". Accordingly, the analog switch 501
outputs not the light power monitor voltage Vpd but the reference
voltage Vref as the output voltage Vpd2. In addition, since the
control signal OFF_APC* is "H", and the sample hold signal S/H* is
"L", the control signal SEL is set to "H". For this reason, the
analog switch 504 sets the hold capacitor 505 in the sample state.
Hence, at the time 421, the reference voltage Vref (=voltage Vpd2)
starts being applied to the hold capacitor 505 via the resistive
element 503.
The reference voltage Vref is applied to the hold capacitor 505 for
a predetermined period Tc (the period from the time 421 to a time
422 in FIG. 4). The period Tc is defined as a period after the
charging of the hold capacitor 505 by the reference voltage Vref
has started until the hold capacitor 505 is charged to the
predetermined voltage Vt. At the time 422, the sequence controller
47 changes the control signal OFF_APC* from "L" to "H".
Accordingly, the control signal SEL changes from "H" to "L", and
the analog switch 504 changes the hold capacitor 505 to the hold
state. As a result, at the time 422, the hold capacitor 505 is
charged to the predetermined voltage Vt by the time constant .tau.
and held at the voltage. After that, at a time 423, the initial
charging operation ends, and the processing switches to execution
of APC of the initial APC mode.
(Setting of Period Tc)
The period Tc will be explained here with reference to FIG. 5.
Referring to FIG. 5, a waveform 511 represents the output voltage
Vpd2 of the analog switch 501, and has a step at time t=0
corresponding to the time 421 at which the voltage switches from 0
to the reference voltage Vref. A waveform 512 represents the
voltage Vsh of the hold capacitor 505 when the reference voltage
Vref is applied to the hold capacitor 505 via the resistive element
503. The hold capacitor 505 accumulates charges as the reference
voltage Vref is applied to the hold capacitor 505 via the resistive
element 503. As a result, the voltage Vsh of the hold capacitor 505
moderately increases with the time constant .tau. defined by the
capacitance C of the hold capacitor 505 and the resistance value R
of the resistive element 503.
The voltage Vsh (waveform 512) of the hold capacitor 505 shown in
FIG. 5 is the step response to the waveform 511 and is generally
given by Vsh=Vref(1-exp(-t/.tau.)) (1)
When the time t at which the voltage Vsh reaches the predetermined
voltage Vt is defined as Tc, Tc is determined depending on the
reference voltage Vref, the voltage Vt, and the time constant
.tau., as is apparent.
The voltage Vt may be designated in advance at a ratio to the
reference voltage Vref. That is, the voltage Vt may be designated
as a ratio x (%) of the light power to the voltage Vt based on the
target light power. In this case, using the ratio x, the period Tc
during which the hold capacitor 505 is charged is obtained by
Tc=-.tau..times.ln(1-x/100) (2) The period Tc can be calculated by
equation (2) using the ratio x and the time constant .tau.. Note
that Tc may be calculated by the sequence controller 47. The
sequence controller 47 switches the control signal OFF_APC* such
that the reference voltage Vref is applied to the hold capacitor
505 during the calculated period Tc.
FIG. 5 shows a case in which the ratio x is set to 80, 90, and
95(%) as an example. Using equation (2), x=80(%),Tc(T80)=1.61.tau.
x=90(%),Tc(T90)=2.30.tau. x=95(%),Tc(T95)=2.97.tau. are obtained.
As can be seen from FIG. 5, when the ratio x is increased, the
period Tc until the voltage Vsh reaches the voltage (0.80Vref,
0.90Vref, 0.95Vref) corresponding to the ratio x becomes long.
Hence, the closer the light power from which light power control by
APC executed after the initial charging operation starts is to the
target light power, the longer the period Tc necessary for charging
the hold capacitor 505 in the initial charging operation is. It is
therefore necessary to set the period Tc within a period assignable
to the initial charging operation. Note that the period Tc
designated by the ratio x is constant independently of the target
light power even when the target light power is changed, as
indicated by equation (2).
(APC of Initial APC Mode and Normal APC Mode)
When the above-described initial charging operation is completed in
the image forming apparatus 100, the processing shifts to execution
of APC of the initial APC mode at the time 423. At the time 423,
the sequence controller 47 switches the signal OFF_LD from "H" to
"L" to start supplying the driving current to each LD, thereby
setting each LD in the turn-on state. At this time, the current
controller 506 in the APC circuit 403 decides the driving current
(switching current value Isw) to be supplied to each LD in
accordance with the voltage Vsh (=Vt) of the hold capacitor 505
charged in the turn-off state of the LD.
The hold capacitor 505 has been charged up to the voltage Vt close
to the reference voltage Vref corresponding to the target light
power by the initial charging operation in the initial APC mode, as
shown in FIG. 4. Hence, the decided driving current has a current
value close to the driving current corresponding to the target
light power. As a consequence, the light power of each LD is
controlled to the target light power in a short time by several
times of APC executed later in response to detection of a BD
signal. Referring to FIG. 4, after the time 423, the sequence
controller 47 sets each LD in the full turn-on state and detects
the BD signal. In addition, the sequence controller 47 switches the
sample hold signal S/H* (H.fwdarw.L) to switch the hold capacitor
505 from the hold state to the sample state at the timing the BD
signal has stably been detected twice. The first APC of the initial
APC mode is thus executed during a period 424, and the voltage of
the hold capacitor 505 approaches the reference voltage Vref
corresponding to the target light power from the voltage Vt
(initial value).
After that, when the APC during the period 424 is completed, and
image formation starts, the image forming apparatus 100 shifts from
the initial APC mode to the normal APC mode. In every scanning of
the photosensitive member 11 by a laser beam output from each LD
(every time a BD signal is detected), the APC operation is
repetitively performed during a predetermined period (periods 425
and 426). In FIG. 4, the voltage Vsh of the hold capacitor 505 is
set to a value sufficiently closer to the reference voltage Vref
during the periods 425 and 426. That is, the light power of each LD
is controlled to a light power sufficiently close to the target
light power, and the light power is considered to have reached the
target light power.
FIG. 4 illustrates only the light emission sequence of one LD. In
this embodiment, the same light emission sequence is executed for n
LDs (LD1 to LDn). As described above, the APC circuits 403 (403-1
to 403-n) are provided for the n LDs, respectively. Hence, the
light emission sequence shown in FIG. 4 is executed for each
LD.
<Procedure of APC in Laser Driving Device 31>
The procedure of the series of APC operations (initial APC mode and
normal APC mode) in the laser driving device 31 described with
reference to FIGS. 4 and 5 will be explained next with reference to
the flowchart of FIG. 6. Note that the processing of each step
shown in FIG. 6 is implemented on the image forming apparatus 100
by causing the CPU (not shown) of the sequence controller 47 to
read out a control program stored in advance in a memory or the
like to a RAM (not shown) and execute the program. The sequence
controller 47 is assumed to start the processing shown in FIG. 6
upon power-on of the image forming apparatus 100 and end the
processing upon power-off.
In step S601, the CPU of the sequence controller 47 (to be simply
referred to as a "CPU" hereinafter) sets the period Tc based on,
for example, an instruction input by the user via the operation
unit (not shown) of the image forming apparatus 100 before the
start of image formation. The period Tc can be set based on
equation (1) or (2), as described above. That is, the CPU controls
the operation unit such that the user can set the ratio x (%).
After that, the CPU advances the process to step S602.
In step S602, the CPU determines whether to start image formation.
In accordance with input of an image formation command, or the
like, the CPU determines whether to start image formation. Upon
determining in step S602 not to start image formation, the CPU
repeats the determination of step S602. Upon determining in step
S602 to start image formation, the process advances to step
S603.
In step S603, the CPU starts the above-described initial APC mode
and also starts the initial charging operation. That is, the CPU
starts the operation of charging the hold capacitor 505 to the
voltage Vt based on the ratio x in the turn-off state without
turning on the lasers. More specifically, the CPU switches the
control signal OFF_APC* to be output to the APC circuit 403 from
"H" to "L", and starts time count from time t=0. In step S604, the
CPU determines whether the period Tc has elapsed after the
switching of the control signal OFF_APC* in step S603 (t.gtoreq.Tc
is satisfied). Upon determining that the period Tc has elapsed, the
CPU advances the process to step S605 to return the control signal
OFF_APC* from "L" to "H". The hold capacitor 505 is thus charged
from the voltage 0 to the voltage Vt (the voltage corresponding to
x % of the reference voltage Vref corresponding to the target light
power).
In step S606, the CPU starts driving the polygon mirror 33 and also
starts supplying the driving current to each of the lasers (LD1 to
LDn), thereby turning on the lasers and setting them in the full
turn-on state. The image forming apparatus 100 thus starts the APC
(of the initial APC mode). When a BD signal is detected as the BD
sensor 36 receives the laser beam from a representative laser, the
CPU starts the APC of each LD in step S607 (period 424 in FIG.
4).
In step S608, the CPU starts supplying a driving current (switching
current) based on the image information to each laser, thereby
stating image formation. The image forming apparatus 100 thus
shifts from the initial APC mode to the normal APC mode. After the
start of image formation, the CPU may execute the APC (of the
normal APC mode) in response to BD signal detection using a laser
beam. In step S609, the CPU determines whether processing
designated by the image formation command is completed, thereby
determining whether to end the image formation. As long as
determining not to end the image formation processing, the CPU
repeats the determination of step S609. Upon determining to end,
the process advances to step S610. In step S610, the CPU turns off
the lasers, and returns the process to step S602. The image forming
apparatus 100 stands by until image formation starts again.
As described above, when performing APC for an LD that outputs a
laser beam corresponding to the driving current controlled based on
the voltage of the hold capacitor, the optical scanning apparatus
according to this embodiment controls the driving current to be
supplied to the LD such that the light power monitor voltage
generated in the charged hold capacitor approaches the reference
voltage from the initial value that is a voltage corresponding to
the amount of charges accumulated in the hold capacitor in advance
at the time of turning on the LD. The hold capacitor accumulates
charges in advance in a state in which the LD is off before the
start of optical scanning of the photosensitive member. When the LD
is turned on, the hold capacitor outputs a voltage corresponding to
the amount of charges accumulated in advance at the time of turning
on the LD, and then outputs a voltage corresponding to the light
power of the LD. The optical scanning apparatus thus controls the
driving current (that is, the voltage of the hold capacitor) to be
supplied to the LD such that the voltage corresponding to the light
power of the LD approaches the reference voltage from the initial
value that is the voltage corresponding to the amount of charges
accumulated in the hold capacitor in advance before turning on the
LD. According to this embodiment, the voltage of the hold capacitor
can approach the reference voltage from a voltage closer to the
reference voltage corresponding to the target light power as
compared to a case in which no charges are accumulated in the hold
capacitor in advance. That is, when executing the APC, the light
power of the LD can be made to approach the target light power in a
shorter time after turning on the LD.
More specifically, the optical scanning apparatus may charge the
hold capacitor to a predetermined voltage close to the reference
voltage corresponding to the target light power before turning on
the LD. When performing the APC, the voltage of the hold capacitor
approaches the reference voltage from the predetermined voltage set
as the initial value. That is, since the light power control of the
LD can be started from the level close to the target light power
after turning on the LD, it is possible to control the light power
to the target light power in a short time.
Note that in the image forming apparatus according to this
embodiment, the target light power is the light power of the laser
beam input to the BD sensor 36. The light power that enters the BD
sensor 36 is desired to be constant. The rising speed and falling
speed of the signal output from the BD sensor 36 depend on the
light power of the laser beam that enters the BD sensor 36. That
is, when the light power that enters the BD sensor 36 changes, the
rising speed and falling speed of the signal output from the BD
sensor 36 change depending on the light power of the laser beam.
For this reason, to always attain the same image write position,
the light power of the laser beam that enters the BD sensor 36 is
desired to be made constant.
On the other hand, the light power of the laser beam to expose the
surface of the photosensitive member 11 to form an electrostatic
latent image on the photosensitive member 11 is controlled in the
following way. The image forming apparatus according to this
embodiment is provided with the potential sensor 30 to measure the
charges on the surface of the photosensitive member 11. The
sequence controller 47 performs control to expose, by a plurality
of light powers of laser beams, the photosensitive member 11
charged by the primary charger 28 at a predetermined timing,
thereby forming a plurality of latent image patterns on the
photosensitive member 11. The potential of each of the plurality of
latent image patterns is detected by the potential sensor 30. The
sequence controller 47 selects a latent image pattern formed with a
predetermined potential out of the plurality of latent image
patterns, and sets the light power of the laser beam corresponding
to the latent image pattern to the light power of the laser beam to
expose the surface of the photosensitive member 11. Note that a
density sensor may be attached to the image forming apparatus, and
the light power of the laser beam to expose the surface of the
photosensitive member 11 may be set based on not the latent image
patterns but toner patterns of a plurality of densities.
The light power control signal included in the control signal S47
is a signal (control coefficient) representing the degree of
control of the light power of the laser beam to scan the surface of
the photosensitive member 11 with respect to the target light
power. The sequence controller 47 outputs the light power control
signal to the current controller 506 of the APC circuit 403. The
current controller 506 controls the switching current Isw such that
the light power of the laser beam to scan the surface of the
photosensitive member 11 is controlled to a light power obtained by
multiplying the target light power (a light power corresponding to
Vref) by the control coefficient.
That is, the image forming apparatus according to this embodiment
controls the light power of the laser beam that enters the BD
sensor 36 to the target light power (first light power). On the
other hand, the image forming apparatus according to this
embodiment controls the light power of the laser beam to scan the
surface of the photosensitive member 11 to form a latent image
pattern on the photosensitive member 11 to a second light power
based on the target light power and the detection result of the
potential sensor.
In this embodiment, for each of the plurality of LDs of the image
forming apparatus of the multi-beam system, the corresponding hold
capacitor is charged to a predetermined voltage in advance before
turning on the LDs. This allows to the light power control to start
from the level close to the target light power for all of the
plurality of LDs. Hence, according to this embodiment, it is
possible to shorten the time necessary until the light power
reaches the target light power by the APC, which is particularly
problematic in the image forming apparatus of the multi-beam
system.
While the present invention has been described with reference to
embodiments, it is to be understood that the invention is not
limited to the disclosed embodiments.
This application claims the benefit of Japanese Patent Application
Nos. 2011-269394, filed Dec. 8, 2011 and 2012-250587, Nov. 14,
2012, which are hereby incorporated by reference herein in their
entirety.
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